A labor for removing axillary buds just before harvesting of tobacco is necessary for securing the yield and quality of leaf tobacco. However, in order to perform the labor by hand work, a huge amount of work is required. Therefore, nowadays, a method involving spraying an inhibitor for tobacco axillary bud growth has been developed and widely used.
As an inhibitor for tobacco axillary bud growth which has been widely used from the Showa 30's (1955-1964), there are known inhibitors including, as an active ingredient, maleic hydrazide or a salt thereof, having penetrating migration characteristics. The inhibitor is actually used at a concentration as high as about 5,000 ppm and is poor in terms of sustained chemical efficacy. Hence, there has been a problem in that a large amount of the inhibitor is required. Further, hydrazine produced by decomposition of maleic hydrazide exhibits oncogenic potential, and hence use of the inhibitor is currently restricted.
Therefore, in recent years, a contact-type inhibitor for tobacco axillary bud growth, which is sprayed by contact with stem, has been used. As the contact-type inhibitor, there are known, for example, an inhibitor including a saturated aliphatic alcohol as an active ingredient and an inhibitor including a dinitroaniline-based chemical substance as an active ingredient.
The inhibitor for axillary bud growth including a saturated aliphatic alcohol as an active ingredient has a high effect of killing axillary buds by contact. However, the inhibitor including a saturated aliphatic alcohol as an active ingredient is poor in terms of sustained chemical efficacy and requires spraying of the inhibitor at least twice in order to prevent elongation of axillary buds in the later growth period. In addition, attachment of the sprayed inhibitor to leaves at the time of use causes bleaching, another harmful effect of the inhibitor, and reduction in quality of the leaves. Further, as the inhibitor is dropped and accumulated at the base part of a plant, all stem bases are necrotized, resulting in killing the plant.
On the other hand, the inhibitor including a dinitroaniline-based chemical substance as an active ingredient is disclosed in, for example, U.S. Pat. No. 3,672,866 A (Patent Document 1), U.S. Pat. No. 4,046,809 A (Patent Document 2) and U.S. Pat. No. 4,123,250 A (Patent Document 3). The inhibitor including a dinitroaniline-based compound as an active ingredient contains a certain organic solvent. The inhibitor has an effect of stopping growth of axillary buds by dehydrating and necrotizing axillary buds by contact of the dinitroaniline-based chemical substance and the organic solvent with the axillary buds and allowing the above-mentioned chemical substance to be absorbed from the axillary bud plumule part or from wounds after removal of the axillary buds to inhibit cell division and has a high effect of inhibiting formation and elongation of axillary buds.
However, the inhibitor including a dinitroaniline-based chemical substance as an active ingredient has the following problems, for example. The inhibitor sometimes causes harmful effects such as deformation of young leaves of the upper node, lack in expansion, necrosis of mesophyll, damages of the petiole parts of middle or upper leaves, developmental disorders of adventitious roots, and necrosis immediately after spraying; and induces diseases such as hollow heart, crown rot, and gray mold from wound sites formed in the petiole base by the harmful effects to cause adverse effects on the yield and quality of leaf tobacco.
An inhibitor comprising a chlorthal-dimethyl-based chemical substance as an active ingredient is disclosed in, for example, U.S. Pat. No. 4,627,869 (JP H05-19521 B; Patent Document 4). Chlorthal-dimethyl has a high effect of inhibiting formation and elongation of axillary buds as with a dinitroaniline-based chemical substance.
Patent Document 4 discloses a method of applying chlorthal-dimethyl to plants followed by an application of fatty alcohols having 6 to 12 carbon atoms, as a known substance which can be used singly as an inhibitor for tobacco axillary bud growth, 5 to 14 days later (sequential application) but it does not indicate experimental data and does not suggest or disclose the improvement in effect of inhibiting the tobacco axillary bud growth of both of the ingredients by the sequential application, either. There has been no document so far, which teaches that the effect of inhibiting the formation and growth of tobacco axillary buds can be synergistically improved through the application of an inhibitor by preparing the formulation by mixing chlorthal-dimethyl and aliphatic alcohol having 6 to 20 carbon atoms, or by preparing the formulation of the both ingredients independently and appropriately mixing the two before the use.
Furthermore, WO 2012/029446 (Patent Document 5) discloses that the effect of inhibiting the tobacco axillary buds can be synergistically improved by combining chlorthal-dimethyl and an ultra-long chain fatty acid synthesis inhibitor.
However, conventional inhibitors for tobacco axillary bud growth including chlorthal-dimethyl as an active ingredient had a high effect of inhibiting the axillary bud but had a low penetration into the plant body, and hence the inhibitor had to be used at a high concentration. Accordingly, it is necessary to use the inhibitor repeatedly until harvest and a large amount of the inhibitor is required. In addition, the inhibitor remains at a high concentration on the surface of the tobacco leaves and hence there has been a problem in that the remaining inhibitor has a harmful effect on the quality of the leaf tobacco.
As mentioned above, the contact inhibitor for axillary bud growth currently used in cultivation of tobacco have many problems yet to be solved from the viewpoint of sustainment of the chemical efficacy and occurrence of harmful effects.
Therefore, a contact inhibitor for tobacco axillary bud growth which is effective in a small amount, is excellent in terms of sustained chemical efficacy, induces no harmful effect, and can contribute to improvement in labor productivity has been desired.